The present invention relates generally to delta-sigma analog signal converters, and more particularly, to an analog signal converter having an delta-sigma modulator and programmability.
One conventional approach for sampling and quantizing a complex envelope of a bandlimited waveform uses a multi-bit analog-to-digital converter and tunable digital oscillator. This approach requires an analog-to-digital converter sampling above the Nyquist rate determined by the highest frequency signal to be converted, followed by a complex frequency translation to baseband and a digital low pass filter. A numerically controlled oscillator supplies the frequency translator with a variable reference and provides for tunability. A fundamental limitation of this approach is that the fidelity of the analog-to-digital converter decreases as the carrier frequency increases. Another drawback is that as the carrier frequency is increased a significant amount of high speed signal processing is required.
Schemes using a bandpass delta-sigma modulator and digital filtering tuned to a fixed frequency and fixed signal bandwidth relative to the sample rate have been described. For example, see H. J. Dressler: "Interpretative Bandpass A/D Conversion--Experimental Results" Electron. Lett. vol. 26, no. 20, pp. 1652-1653, Sep. 27, 1990, R. Schreier and M. Snelgrove, "Decimation for bandpass sigma-delta analog-to-digital conversion", Proc. Int Symp. Circuits Syst., vol. 3, May 1990, pp. 1801-1804, and G. Troster, et al, "An interpolative bandpass converter on a 1.2 .mu.m BiCMOS analog/digital array", IEEE Journal of Solid State Circuits, pp. 471-477, April, 1993.
Accordingly, it is an objective of the present invention to provide for an improved analog signal converter having a delta-sigma modulator and programmability.